1. Field of the Invention
This invention relates to an idling speed control system for an internal combustion engine.
2. Description of the Prior Art
Generally it is preferred to maintain the idling speed of the internal combustion engine of a vehicle at a low speed of about 600 to 700 rpm from the viewpoint of fuel consumption and stability of the combustion in the engine. Thus, there have been known various idling speed control systems in which an actuator for controlling the opening angle of the throttle valve or the opening angle of a bypass valve in a passage bypassing the throttle valve is provided and the actuator is controlled according to the difference between the actual idling speed and a desired idling speed determined according to the operating conditions of the engine such as the temperature of the coolant and whether or not a cooler is in operation so that the actual idling speed is equalized to the desired idling speed.
In the conventional idling speed control systems, in order to improve response speed, the throttle valve or the bypass valve is first moved to a temporary position which is expected to be suitable to obtain the desired idling speed, and then feedback control is effected to finally converge the corrected actual idling speed, which has substantially approached the desired idling speed, to the desired idling speed. See Japanese Unexamined Patent Publication No. 55(1980)-156227, for example.
In the system in which the idling speed is controlled in the manner described above, the throttle valve or the bypass valve must be moved by a large amount to the temporary position when, for example, the cooler begins to operate during idling and the desired idling speed is changed. In order to limit fluctuation in the idling speed in such cases, it is naturally desired that it be possible for the actual idling speed to be equalized to the desired idling speed by one movement of the throttle valve or the bypass valve. Thus there has been proposed a method of controlling the idling speed in which the relationship between the position of the control valve (the throttle valve or the bypass valve) and the idling speed is stored in detail in a memory in advance and the control valve is driven to control the idling speed according to the relationship stored in the memory. However, this method is disadvantageous in that the actual idling speed cannot be always precisely equalized to the desired idling speed since the performance of the engine differs from engine to engine even in the same type engines due to manufactural differences, change with time and the like, and accordingly the idling speed may possibly fluctuate for a while.